1. Field of the Invention
The present invention relates to submarine laser communications (SLC), and more particularly to secure two-way communication techniques providing high quality communications in real time between an aircraft and submerged submarines at operational depths and speeds.
2. Description of the Prior Art
From the time that the first submarine slipped beneath the ocean surface, there has been a desire for two-way (duplex) communications with them. Since that first submarine, communication technologies have substantially improved basic submarine communications; however, for various reasons no one system has been completely satisfactory. Current methods of communication generally require the submarine either to surface directly or to send a probe to the surface, neither of which is very desirable. Such action potentially exposes the submarine to its adversary, it limits the submarine's overall maneuverability when in use, and it detracts from the submarine's mission. Because a submarine is most vulnerable when it attempts to communicate, the present solution when far from home is basically no communication at all.
Of the present systems, one (known as an ELF system) uses an extremely-low frequency modulated carrier, and a second uses very-low frequency (VLF) carrier signals. The ELF system with transmitters in Wisconsin and Michigan uses the skin effect of the earth to send very high power signals to distant submarines at relatively shallow depths; however, the system is characterized by extremely low data rates (on the order of minutes per character), is unidirectional, is non-selective, and at present requires the submerged vessel to trail a long antenna wire to receive the signals. Its main advantage is that it is a completely covert system, i.e., secure), and a submerged submarine can receive ELF signals to depths of several hundred feet whether in open water or under an ice pack. The VLF system, unlike the ELF system, has usable data rates for message traffic delivery, but a communicating submarine must be close to the surface (within 10 meters) to receive a transmission. If the high-power shore-based antenna is inoperable, a transmitting airplane must trail a long antenna wire (the TACAMO system) to communicate over a significant ocean area. A submarine's vulnerability is increased during the period of communication. Like the ELF system, the VLF system provides only one-way communications. This combined with their low message delivery rate makes them undesirable for tactical operations.
Communication satellites have also been used (e.g. SSIXS) to provide a form of two-way communications with submarines. Such satellite systems overcome numerous shortcomings of the VLF and ELF systems, but suffer from the submarine's need to surface an antenna to communicate, which dictates that a submarine must be at least at periscope depth to communicate or float a buoy on the ocean surface. To minimize this period of vulnerability, a form of burst communication is utilized. Even with burst communication the potential for detection of the uplink remains high, and therefore, this technique does not offer truly covert operation. Other submarine communication systems use slot buoys having built-in UHF transmitters. The submarine releases the buoys underwater to float to the surface where a prerecorded message is then transmitted generally after a delay of many minutes. When the transmission is complete, the surface buoy self-destructs and sinks to the ocean bottom. As with the VLF and ELF systems, this method of communication is one-way and is often quite delayed to limit the exposure of the submarine. Both the releasable buoys and the satellite transmission suffer from the disadvantage of potentially revealing the approximate position of the submarine.
None of the aforementioned communication techniques has any IFF (identify, friend or foe) capability when the submarine is at operational depths and speed, and all of the transmissions can be intercepted and/or jammed by an adversary. The consequence of these limitations is that the submarine has limited utility for coordinated activities, and moreover, it must constantly be wary of being mistaken as an adversary by a member of its own naval group. In effect, present submarine communication systems inhibit the submarine from simultaneously performing its mission and communicating with its own naval group, which results in limiting the tactical missions of submarines.
Over the last 10 to 15 years various suggestions, studies and field experiments have been done to show the feasibility of using lasers of suitable wavelengths to communicate between submarines and aircraft and/or satellites. Blue or blue-green lasers have been chosen because light transmission through water is best at or near 490 nm. Transmission falls off rapidly below 400 nm and above 550 nm. Although satellite-to-submarine communications has been suggested (e.g., see U.S. Pat. No. 4,764,982), such systems are currently impractical using existing technology.
A tactical and covert two-way communications channel between an aircraft and a submarine operating at operational depths and speeds would provide: (1) very useful and timely information, for example, over-the-horizon target information; (2) coordination of ASH maneuvers between an aircraft and a submarine; and (3) coordination of a surface fleet and submarines in a direct or associated direct support role. A covert communications system would preferably include an IFF capability so that a submarine could identify itself to a supporting aircraft without revealing its location to an enemy, and the aircraft could positively identify the fleet submarine and eliminate the confusion between a friendly asset and an adversary.
This invention is directed to practical techniques for providing secure laser communications which overcomes these problems.
A general object of the invention is to provide a secure method of communicating with submerged platforms, such as submarines, and one that is capable of operating in all weather conditions.
A further object of this invention is the provision of a communicating technique that will allow two-way communications between an airborne platform and a submarine at operational depths and speeds.